Traditionally wind turbines have been placed in rural areas where they can take advantage of strong winds with a low turbulence level. The realization of the need for generating energy from sustainable sources expands, there is more interest in smaller wind turbines for use in areas not conventionally considered as suitable, such as urban and peri-urban zones.
The urban environment has unique challenges to the wind turbines. First of all, the overall wind speed is lower than the wind speed in open rural areas. A wind turbine for use in an urban area should therefore have a low cut in speed, i.e. the lowest wind speed at which the wind turbine begins producing usable power, and a low rated wind speed, i.e. the speed at nominal operating conditions. Furthermore, the presence of buildings and other structures increases the turbulence level of the flow. Considering that the wind energy capture of a wind turbine varies with the cube of the velocity of the wind, a machine designed to operate under certain conditions may, within seconds, undergo stresses much greater than those for which it was designed.
Also, since free space in urban areas is limited, turbines are commonly mounted on buildings. But most buildings are not specifically designed for supporting wind turbines, and are not suited for supporting heavy constructions or absorbing vibrations.
Manufacturing machines robust enough to withstand turbulent wind flows and maximum wind flows during storms makes wind turbines costly. Furthermore, the robust type machines tend to get very heavy and are therefore less suitable for mounting on buildings.
It is also known to provide wind turbines with a system which feathers the blades of the wind turbine, i.e. pitches the blades so as to reduce their lift capacity to reduce the speed of the turbine or to even shut down the turbine during high wind speeds. The known systems are complicated and elaborate, and are therefore unsuited for cost efficient small wind turbines. However, small wind turbines without a feathering system suffer from severe wind loads during storms, which asks for heavy support structures of the rotor. Such small wind turbines are therefore also costly.